Evolution and Ecology – Chapter 2

What is Evolution?• Biological Evolution can be considered a

change in attributes within a population over time.– Specifically, it is a change in gene frequency– Evolutionary changes lead to adaptation

(maximizes fitness)

• What drives adaptive evolution?– Genetic drift (founder effect, bottleneck)?– Allopatric speciation (physical separation of

two populations)?– Natural selection?

What is Necessary for Natural Selection to Operate?

• Variation must occur within a population– Breeding domestic animals

• An excess of offspring must be produced• Not all individuals can survive to

reproduce– It’s a mean old world!

• Only those best able to garner limited resources will survive and reproduce

• Characteristics must be inheritable and more frequent in the next generation

Gypsy Moth Phenotype Selection

Light

Dark

• Soot from the industrial revolution caused light colored trees to become dark.

• A decrease in the number of light moths and an increase in the number of dark moths was observed.

• Could it be that light colored moths were more vulnerable to predation?

Dark form

Light form

The proportion of light to dark colored moths has been changing since about 1950. The trees have become noticeably lighter as well!

We have seen the same thing in the United States!

Natural selection acts on phenotypes – the observable attributes of individuals (remember AA and Aa have the same phenotype).

Although adaptive evolution is a change in genotype frequency, it is much easier to observe natural selection directly on the phenotype.

Three Types of Selection(You should be able to describe these)

Original Distribution

Before Selection

After Selection

Directional Selection

Stabilizing Selection

Disruptive Selection

Directional Selection- Probably accounts for most phenotype changes found in the wild.

A severe drought from 1976 -1978 caused an 85% drop in the population.

Only those with larger beaks could eat the large seeds.

Resistance to pesticides can also result in Directional Selection.

Important

Stabilizing Selection- phenotypes near the mean are more fit than those at the extremes; most common ecological situation.

Human Birth Weight

More Stabilizing Selection: Lesser Snow Geese

Safety in numbers.

Relative Hatch Date = mean date that hatching occurred.

In this case, it is best to hatch when there are many others around.

Disruptive Selection:not very common

• Extremes are favored over the mean

• Unless some form of reproductive isolation occurs, extreme phenotypes may continue to mate and produce intermediate phenotypes

Overall: Organisms are adapted to their environment!

Four Constraints to Adaptation• Genetic Forces– Mutation – usually detrimental – Gene flow – immigrants can smooth out local

adaptations• Environments are Continually Changing– Most significant short-term constraint

• Adaptation is a Compromise– A loon’s wings are efficient for diving, but not

flying• Historical Constraints– Organisms have a history and change in small

increments

Case Study: Clutch Size in Birds• Clutch = number of eggs laid and differs

among species

• Clutch size can be affected by proximate (functional; physiological) factors but is a result of ultimate (evolutionary; genetic) factors.

• Clutch size does not always = maximum physiological number

Species Normal Maximum

Mallard ?? 100

Herring Gull 2-3 16

Yellow Shafted Flicker 6-8 71

House Sparrow 3-5 50

Determinate layers do not vary the number of eggs they produce, indeterminate layers do:

Ovulation is usually stopped before the physiological maximum number of eggs are produced.

David Lack (1947) put forward the idea that clutch size was determined by the number of young the parents could provide food for.

If this is true, then the highest production of young should be the normal clutch size (optimal size).

A cost benefit analysis supports this idea.

Clutch Size

Optimality Model

No organism has an infinite amount of energy to spend on its activities!

If additional young are placed in a nest, all young will suffer if there is not enough food.

Normal clutch size = 11.

In the tropics, small clutch size is typical.

Smaller clutch size = less parental time away from the nest lower predation rate.

Therefore, low clutch size in the tropics is believed to be an adaptation to predation levels.

Maximum production.

Not All Species Follow Lack’s Hypothesis:

Normal clutch = 3 - 4 However, what is not known is the effect on the parents – can they survive until next breeding season? Or does raising more young exhaust them?

Other - gene flow from different habitat qualities.

Coevolution – Specific and ReciprocalOriginally described the reciprocal evolutionary

influences that plants and plant-eating insects have had on each other (Ehrlich and Raven 1964).

Predator prey relationships – ‘arms race’

What Is Necessary For Natural Selection To Work?

• Ability to replicate• Produce an excessive number of units

above replacement needs• Survival depends on some attribute (size,

color, behavior)• Attributes must be transmittable to the

next generationAn individual meets these requirements, but other units do also: gametic, kin, and group.

Units of Selection• Gametic Selection (e.g. sperm mobility) – does

not directly impinge on ecological relationships.

• Kin Selection (you stole from my kin!) – increase survival of related individuals b/c they share many of your genes.– helps to explain some altruistic behavior

• Group Selection – can occur when populations are broken up into discrete groups.– Groups with less adaptive genes may go extinct– Highly controversial– Bird reproduction is limited so that they do not

overpopulate an area